1. Field of the Invention
This invention generally relates to a print ribbon cassette for use in various types of printers and typewriters, and, in particular, to a print ribbon replaceable cassette employing an elastic driving belt for carrying out ribbon feed.
2. Description of the Prior Art
FIG. 3 shows a typical prior art print ribbon cassette 10 capable of replacing used print ribbon with fresh print ribbon. As shown, a print ribbon 12 is wound around a supply reel 11 thereby forming a supply roll 13 of print ribbon 12 as housed inside of the cassette 10. A fan-shaped supply plate 14 is pivotally supported on a base plate 10a of the cassette 10 so as to be pivotal around a roller shaft 15a of a roller 15, and the supply reel 11 is rotatably supported on the fan-shaped supply plate 14. The print ribbon 12 is lead out of the supply roll 13 and passed around a pair of guide rollers 16, 16 mounted on the base plate 10a of cassette 10, and, then, the print ribbon 12 extends outside of the cassette 10 and, after passing around another pair of guide rollers 17, 17, it is wound around a take-up reel 18 so that a take-up roll 19 is formed around the take-up reel 18. The take-up reel 18 is rotatably supported on a fan-shaped take-up plate 20 which, in turn, is pivotally supported on the base plate 10a of the cassette 10 so as to be pivotal around a roller shaft 21a of a roller 21.
As shown in FIG. 3, the rollers 15 and 21 are freely rotatably mounted on and located at respective corners of the base plate 10a of the cassette 10 and a drive roller 22 is also provided as rotatably mounted on and located near the center of the cassette 10 so that a triangular shape is defined by these three rollers 15, 21 and 22 with one side extending in parallel with and adjacent to the bottom side of the base plate 10a of the cassette 10. As shown, the rollers 15 and 22 are located on opposite sides of the supply roll 13, and the rollers 21 and 22 are located on opposite sides of the take-up roll 19. The drive roller 22 is coupled to a driving mechanism (not shown) of a printer when the cassette 10 is detachably mounted on the printer so that the drive roller 22 may be driven to rotate. An endless, elastic belt 23 extends around these three rollers 15, 21 and 22 and is in pressure contact with the supply and take-up rolls 13 and 19. The drive roller 22 is driven to rotate clockwise so that the endless belt 23 advances in the direction indicated by the arrows a to carry out ribbon feed.
In the case of the print ribbon cassette 10, when a top cover (not shown) is removed, the fan-shaped plates 14 and 20 may be pivotted outwardly as indicated by the one-dotted lines thereby releasing the pressure contact between the endless belt 23 and each of the supply and take-up rolls 13 and 19 whereby replacement of print ribbons may be carried out with ease. However, after print ribbon replacement, when the fan-shaped plates 14 and 20 are pivotted back to their original positions inside of the cassette 10, the supply roll 13 tends to rotate counterclockwise and the take-up roll 19 tends to rotate clockwise due to friction with the endless belt 23 so that there is produced a slack in the print ribbon 12 as illustrated in FIG. 4. When such a slack is produced in the print ribbon 12, it cannot be eliminated easily.